The primary structure of neuromedin U from the rat ileum was established as: Tyr-Lys-Val-Asn-Glu5-Tyr-Gln-Gly-Pro-Val10-Ala-Pro-Ser-Gly-+ ++Gly15-Phe-Phe-Leu- Phe-Arg20-Pro-Arg-Asn.NH2. There was no evidence for microheterogeneity. This amino acid sequence contains two deletions and nine substitutions compared with the neuromedin U-25 from the pig. In particular, the replacement of the Arg16-Arg17 processing site in the porcine peptide by Gly14-Gly15 in the rat means that a peptide corresponding to neuromedin U-8 was not found in the rat intestine.